Rituximab 375 mg/m(2) weekly x 4 has been reported to induce a 60% response rate in patients with relapsed follicular lymphomas (FL). Our aim was to examine the effect of this rituximab schedule on circulating FL cells in an ongoing multicenter study. One hundred fifty-four patients with FL were examined by nested polymerase chain reaction (PCR) at baseline for the presence of t(14;18) translocation-carrying lymphoma cells in bone marrow and/or blood. Sixty-four patients (42%) had PCR-detectable t(14;18)(+) FL cells. Pretreatment characteristics of these 64 patients were as follows: one had stage I, nine had stage II, 14 had stage III, and 40 had stage IV disease. Thirty-five patients had bulky disease (> or = 5 cm) and 25 patients had an elevated serum lactate dehydrogenase (LDH) level. Bone marrow was morphologically assessed in 64 patients, and 39 of these patients had an infiltration with FL cells. Blood samples from 51 patients were available for PCR analysis between weeks 8-12 after induction therapy, and 28 of these patients (55%) were PCR negative. Paired blood and bone marrow samples were available for PCR analysis from 39 patients between weeks 8-12 after induction therapy with rituximab. Thirteen of these patients (33%) did not have PCR-detectable cells in blood and bone marrow, while 26 patients (67%) still had circulating t(14;18)(+) cells in either bone marrow (eight patients), blood (one patient), or both (17 patients). PCR negativity in blood and bone marrow in 13 patients was statistically significantly associated with partial or complete response after induction therapy with rituximab (P = 0.006). However, clearance of PCR-detectable t(14;18)(+) cells in bone marrow and/or blood could not be associated with any low tumor burden pretreatment characteristics such as stages I/II, absence of morphological bone marrow infiltration or tumor bulk of > or = 5 cm, and normal serum LDH.